Estimating E savings from air sealing in huge leaky house

I'm asked to estimate the heating energy savings from air sealing a 10,000 sf house built in 1900. The former mansion has 6 undampered chimneys and about 40 windows needing repair as well as doors, some needing replacement and all needing weatherstripping. The house has a semi heated basement, and three stories on top of it.

The house has new radiators in all rooms and a 92% efficient boiler.

How do I estimate the potential heating savings from all the air sealing possible here?

The most accurate way would be to have a blower door test conducted. Once you know how much infiltration you have you should be able to determine the total energy waste associated with that air infiltration.

Thanks! That is exactly the information I needed to make sense of the blower door data.

Now I need to learn how to estimate the potential energy savings from changing out a 23 y.o. boiler that was 80% efficient when new. The replacement would be a 95% boiler potentially coupled with solar hot water. There are 25 people here who take showers daily.

Also, I've read about heat exchangers to recapture waste heat from the pipes running from showers. "EcoDrain" is one such unit. Does anyone have experience with these? And if so what type of efficiencies are they finding?

If you know the annual fuel heating energy bill you can make an estimate. If you know the cost per therm of natural gas or per gallon of fuel oil you can estimate the total amount of energy required. For example, for a 80% efficient propane unit and if we assume 340 gallons of propane for heating...340 * 91,300 * 0.8 = 24.8 MMBTU produced. We then divide this by 0.95 and 91300 for the new boiler to get 286 gallons required for the new furnace. (91300 is the # of BTU's in 1 gal of propane). I hope you see my logic... Or just divide .8 by .95 to get the ratio of new energy required (84%), 16% less energy.